Allow split.adjust = list(scales = "free") to work as intended

[dtm2451]

PR utilizes knowledge that I did not have when originally writing code for most min / max inputs -- that you can give limits = c(<some_min>, NA) or c(NA, <some_max>) to have ggplot fill in the NA side, at plot time, based on the min/max of the plot's/facet's data--both to simplify these code chunks & to allow better control of faceting by letting split.adjust work as intended, re: #114

For example, split.adjust = list(scales = "free") could now be used in dittoPlot() to let facets zoom in on their own data:

library(dittoSeq)
example(importDittoBulk, echo = FALSE)
# without 'split.adjust'
dittoPlot(object = myRNA, var = dittoSeq:::.extract_Reduced_Dim("tsne", 1, myRNA)$embeddings, group.by = "timepoint",
    split.by = c("clustering"))

# with 'split.adjust'
dittoPlot(object = myRNA, var = dittoSeq:::.extract_Reduced_Dim("tsne", 1, myRNA)$embeddings, group.by = "timepoint",
    split.by = c("clustering"),
    split.adjust = list(scales = "free"))

ToDo:

• Test a bit more extensively.
• Check for stale documentation

[dtm2451]

Before I merge, just want to note the other functions which benefit from this PR giving them better compatibility with faceting controls like split.adjust = list(scales = "free"):

• dittoPlot() + derivatives (dittoBoxPlot(), dittoRidgePlot(), dittoRidgeJitter(), multi_dittoPlot())
• dittoFreqPlot() <-- probably where this update becomes most useful, specifically for lower frequency cell types
• dittoPlotVarsAcrossGroups()

Other functions where min/max code was updated, but targets the color scale, so has no direct effect on split.adjust = list(scales = "free") and the like:

• dittoScatterPlot()
• dittoDimPlot()
• dittoDotPlot()